MQTT: add JSON payload corresponding to IEC Structured types

- uses https://github.com/cesanta/frozen
- frozen.c is added as a resulting c file in generat_C (fix toolchain_gcc.py as it was breaking dependency checking)
- structured types are converted to C #define, in turn generating json_scanf and json_printf statements

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                  <xsd:element name="OSX">

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                    <xsd:complexType>

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                      %(toolchain_gcc)s

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                    </xsd:complexType>

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                  </xsd:element>